Vortex tube with divergent hot end



July 7, 1959 l W p, HENDAL 2,893,215

VORTEX TUBE WITH DIVERGENT HOT END Filed June 5, 1956 4o o di u. .30 m mx -z," ET y? o II 8E I o .lo I sf l TOTAL' GAS FIG. 4

INVENTOR:

wlLLEM PIETER HENDAL BY: Mu

HIS ATTORNEY nited States Patent i 'i 893,215 lvolrrrix TUBE WITH DIVERGENT Hor END Willem Pieter Hendal, Amsterdam, Netherlands, assignor :to lShell Development Company, New York, N.Y., a

'corporation of Delaware This invention relates to a vortex tube within which a gas stream is expanded `with gyratory motion about the tube axis to separate the gas into hot and c old fractions, and from which tube g'as is discharged continuously in a manner dependent upon the particular purpose to which Athe tube is put. Thus, the two gas fractions maybe discharged in any desired ratio from zero to infinity; in other words, two gas streams having different temperatures or only one gas streammay be discharged from the tube. The utility of discharging only one stream will become apparent from the sequel. `Although not restricted thereto, vortex tubes find particular utility for the production of cold gas streamsfrom gas which occurs initially Vat superatmospheric pressure, especially when the refrigeration load in a given process wherein the cold gas is to be utilized is so small or `intermittent that more elaborate machinery, such as adiabatic expansion engines, are noteconomically attractive.

The Vortex tube, 4also known as the Ranque tube or Hilsch tube, uses no moving parts and is low in cost. -Such a'tube was `described in U.S. Patent No. 1,952,281 toRanque, and Hilsch drew further attention to it in articlespublished in Zeitschrift fr Naturforschung, vol. 1, pp. 208-214 (Wiesbaden, Germany, 1946) and in Review lof Scientific IInstruments,`vol. 18, pp. S-113 (New York, 1947). A bibliography on vortex tubes, written by Curley and MacGee, Jr., was published in Refrigerating Engineering, vol. 59, 1951, pp. 66 and 191-193. As is set out in the said patent andthe other publications, the vortex tube includes atvortex `chamber having the shape of a body of revolution, such as a cylinder, an inlet for admitting Va feed gas under 'pressure with a gyratory motion, e.g., one or more inlet pipes situated near one end of the v-tube and disposed tangentially and in the same circumferential direction, and suitable outlet means for discharging one or both of the gas fractions which appear within the tube. These fractions are formed respectively as a peripheral stream in contact with the inner surface of the tube wall and as a core rotating coaxially within the peripheral stream, the former being warmer and the latter `being colder than-the feed gas. Both fractions have pressures lowervthan that of the feed gas but the pressure `of the fraction having the higher temperature exceeds that yof the otherfraction, in accordance with the Ranque effect. This phenomenon of the separation of the gas by expansion within the vortex tube into hot and cold fracftions is hereinafter referred to as the heat-separation effect. This heataseparation `effect results in the heating of atleast apart of the vortex tube wall, unless it is ,extraneously cooled.

The. dimensions of such tubes may vary lwithin wide Alimits :according rto the conditions under which the tube amplesgiven 'in' the art noted above. Thus, the vortex tube 2,893,215 Patented July 7,- 1959 2 may have a length of'6 to 30 diameters, length of 18 to 20 diameters being common.

Various outlet arrangements are possible. In one com` mon arrangement, the cold gas fraction is discharged through an orifice situated at the tube axis in aV plate or end wall near and ,to one side of the tangential inlet; the vortex tubeextends through almost all of its length t the other side of the inlet, and the latter part, also known as the hot end, has a discharge opening for the hot gas fraction, situated either near the periphery or near the axis. By throttling one or the other of the efuent streams, e.g., by the use of a throttling valve at the holtgas outlet and/ or by selecting a cold-gas orice of suitable size, their ratio and, hence, their temperatures can be varied; typical temperature effects are indicated graphically by ,Hilsch,- op.cit., and by Sprenger, on page v295 of Zeitschrift fr Angewandte Mathematik und Physik, vol.l II (Basel, Switzerland, 1951). Hot gas which travels away from the inlet through the hot end in excess of the amount, if any, which is discharged at the extremity of the hot end tiows again with the cold core toward the discharge oriiice for the cold vgas fraction. By cooling the hot fraction while within the vortex tube, as by applying a coolant to the wall of the hot end of the tube (see German Patent No. 468,487), `the volume of gas that can be drawn olf as the cold fraction at a given temperature is increased and/or the temperature of the discharged cold fraction can be decreased, and a cooled gas stream is attained even when the hot end ofthe tube is fully closed, so that all gas is discharged as cold gas; the converse is true when the core of the vortex contains a central heating element, such as a small tube through which a heating iluid is circulated, making it possible to draw olf all or most of the gas as hot gas by greatly restricting or completely closing off the central cold-gas orilce. In either case an increased heat-separation within the tubes promotes heat exchange with the extraneous cooling or heating fluid.

Among still other draw-olf arrangements, to which the present invention is also applicable, are that using the socalled uniow principle, in-which .the outlets for the cold and hot gas fractions are arranged concentrically at or near the extremity of the hot end, and that using an annular opening surrounding the Vtube axis `at either end of the tube as the cold-gas outlet; these are illustrated in the Ranque patent cited above.

In all of the foregoing variants, it is desirable to produce a high degree of heat-separation within the vortex tube, and it is the object of this invention to provide a vortex tube having an improved shape by which this heat-separation is increased.

In summary, according to the invention the vortex tube has a hot end, situated to one side of the gas inlet, which end diverges away from the inlet at an angle between 2 and 6, preferably between 3 and 4, and a discharge arrangement of any suitable type, such as those outlined above, is provided so that the gas which is admitted through the inlet with a gyratory motion is separated into hot and cold vfractions of which the former moves as a peripheral stream in contact with the inner surface of the diverging part of the tube. This part of the tube may be cooled, as by providing a jacket through which a cooling liquid is passed; although the tube Wall and peripheral gas stream may bethereby cooled to below the temperature of the feed gas, they are nevertheless for convenience herein called the hot end and the hot gas fraction, respectively, to distinguish them from other parts of the tube and from the cold gas.fraction, which is at a lower temperature.

The invention `will be described in detail with reference to the accompanying .drawings forming a part of this spec- .n ication and illustrating one specific arrangement of the vortex tube by way of example, wherein:

Figure 1 is a longitudinal sectional View of a vortex tube constructed in accordance with the invention;

Figures 2 and 3 are transverse sectional views taken on the correspondingly numbered lines of Figure 1; and

Figure 4 is a graph showing the heat-separating eects achieved by vortex tubes of various shapes.

An understanding of the heat-separation effect is basic to an understanding of the following description and the improved results attained. Accordingly, a summary thereof is presented by way of introduction to the detailed description.

A theoretical explanation of the heat-separation effect is based on the fact that there is evidentlya transfer of energy from the gas at the central part of the tube to the gas near the conlining wall. This transfer of energy increases the temperature of the outer gas layers leaving the tube as the hot gas fraction, e.g., through the throttle valve, and lowers the temperature of the inner gas layers escaping as the cold gas fraction through the orifice.

Closer examination shows that there is rst a transfer of energy caused by the friction between the layers of the rotating gas column which are in the neighbourhood of the tube axis and the layers nearer to the wall of the tube. As the inner layers rotate at a higher angular velocity than the outer layers, the inner layers tend to accelerate the outer ones by friction, the result being a transfer of kinetic energy in a direction outward from the tube axis. A second, more important contribution to the temperature effect is the transfer of energy caused by turbulent currents in the radial pressure eld of the rotating gas column. Owing to the strong centrifugal forces there is a considerable difference in pressure between the inner and outer layers. As a result of the said turbulent currents a quantity of Igas will move outwards and be compressed more or less adiabatically, as a result of which the temperature of the outer gas will rise. A quantity of gas moving in the opposite direction will expand and become cooler. In this way the turbulence will cause a temperature distribution in a radial direction.

According to this theory, the Ranque effect is governed by three variables, viz.:

(l) The elfect is dependent on the absolute temperature (T.) of the fed gas introduced.

(2) The effect is dependent on the ratio of the inlet pressure of the feed gas (p) to the expansion pressure outside of the tube (p); this pressure ratio is hereinafter termed the effective pressure ratio.

(3) The effect is dependent on the ratio between the specific heat at constant pressure (cp) to that at constant volume (cv) of the gas used.

In order to compare quantitatively the eiect of vortex tubes of different designs, when the tubes in question are used as a cooling apparatus, note is taken of the ratio between the decrease in the enthalpy of the quantity of gas leaving the vortex tube as cold gas and the decrease in the enthalpy which would be theoretically reached when the whole amount of gas were allowed to expand isentropically while carrying outexternal work. This ratio is hereinafter termed the cooling eifect; it may be considered as the eiciency of the vortex tube.

In the case of an ideal or practically ideal gas such as air (wherein the ratio is constant and there is a very small or no I oule-'Ihomson elfect) the above-mentioned cooling eicel; Q1 elliency may be expressed as follows, based on a calculation. of this ratio:

[LAT

n=the separation coecient, that is, the ratio of the quantity of the cold gas fraction to the total quantity of gas; AT=diiference in temperature between the feed gas and the cold gas leaving the vortex tube; f nr=the cooling elect or eiciency factor of therefrigeration; Y T0=absolute temperature of the feed gas; 17a-:expansion pressure outside of the vortex tube; p0=pressure of the feed gas supplied; and k=cp E,

In Hilschs experiments, in which the vortex tube had a diameter of 4.6 mm. and a cold-gas orifice of 2.6 mm;y a maximum cooling effect correspond to a value of ,u.=0.6. In this case the quantity 17, had a value of near ly 0.18. In his publications Hilsch observed that the cooling effect was not inuenced by any special shape of the vortex tube or of the hot end thereof. Sprenger, op. cit., states on page 297 that the cooling elect is only slightly affected by the shape ofthe vortex tube and mentions generally, inter alia, divergent and convergent tubes.

Contrary to these earlier opinions, it has now been found that, under conditions which are otherwise comparable, appreciably lower temperatures or a signicant- 1y higher heat-sepaartion eifect can be obtained by constructing the hot end of the vortex tube with a divergence away from the inlet, provided that the angle of divergence is held between 2 and 6, preferably between 3 and 4; outside of these limits only the slight etfectsnoted by prior experimenters are observed. By the angle of divergence is herein meant the apex angle of a cone the surface of which coincides with the divergent part of the tube; this part is frusto-conical and should have a length of at least one diameter thereof, preferably at least four times the initial or smaller diameter thereof.

Referring to Figures l-3 in detail, the tube may include a tangential inlet nozzle 29 the end 30 of which is advantageously joined to the conning wall of the vortex tube by a spiral wall 31, as shown in Figure 2, so as to center the resulting gas vortex at the axis of the tube. The tube is shaped internally as a surface of revolution, and includes a section 32 which diverges away from the inlet at an angle between 2 and 6, preferably between 3 and 4. The wide end of section may be joined smoothly to a cylindrical section 33. The sections 32 and 33 (when provided) constitute the hot end of the tube. An orifice disc 34, mounted near the inlet nozzle, has an orice 35 situated at the central axis of the vortex tube for the discharge of the cold, central part of the gaseous vortex which constitutes the cold fraction. The latter flows through a short diverging section 36 into a discharge pipe 37. The hot end of the tube may have a convergent section 38 by which the hot fraction is led to a discharge pipe 39 and with a suitable throttling device, such as a valve 40 which s axially reciprocable. The tube is jacketed as shown at 41, whereby a cooling liquid such as water can be circulated through the inlet and discharge nozzles 42 and 43 and in contact with the conning `walls of the sections 32 and 33 for cooling these walls intensively. This cools the hot gas fraction, which moves as a peripheral stream within the tube in contact with the confining wall sections. This cooling may be suciently intensive to cool the discharged hot gas fraction approximately to or even below the temperature in which:

of the initial gas. Adjustment of the valve 40 permits the ratio of the hot and cold gas fractions to be varied; when the valve is fully shut all entering gas is discharged as cold gas through the orifice 35, but there is nevertheless a separation within the tube whereby a hot gas fraction travels through the hot end, before returning toward the orifice near the central axis of the tube.

Example The advantages of the construction according to the invention are indicated by the following tests, performed using air in four different vortex tubes, one being cylindrical throughout and the other three having different angles of divergence; these tubes are herein referred to as I, II, III and IV, respectively.

The chief dimensions of the cylindrical vortex tube I were:

Inside diameter mm 10 Length do-- 190 Area of inlet nozzle at 30 sq. cm 6.72 Diameter of discharge oriiice 35 mm-- 6 Pressure of the feed gas, p -6 atmos. abs. Temperature To of the feed gas ..-519 R. (59 F.). Rate of air ow 61.7 lbs/hr. External pressure p 1 atmos. abs.

In these tests various separation coefficients were used for each tube by adjustment of the throttle Value at the hot end outlet. The test results are shown in Figure 4 of the drawing, wherein the cooling effect is plotted against the separation coeicient, the curves being identified with Roman numerals corresponding to the vortex tubes used. The curves show that the optimum cooling effect is obtained with an angle of divergence of about 3.6 and a separation coeicient of 0.65. Compared to the cylindrical vortex tube I, there is a improvement in the cooling eiect.

No extraneous wall cooling was used in the foregoing which were practically adiabatic tests. In additional tests with tubes shaped as tubes I and III, above, other conditions remaining unchanged, water at a temperature of 59 Was circulated through the jacket 41 which surrounded the hot end of the tube; results for these tests are also shown on Figure 4 by dashed lines marked V and VI, respectively. These curves show a maximum cooling effect for tube VI, having an angle of divergence of 3.6, when the separation coeicient was 0.80; in this instance there was an improvement in the cooling effect of compared with the cylindrical tube. These results show the peculiar merit of combining the diverging tube with the means for cooling the tube wall.

I claim as my invention:

l. A vortex apparatus for separating a gas stream therein into hot and cold fractions by the heat-separation effect comprising: a chambered body enclosing a tubeshaped vortex chamber at least a part of which diverges with an angle of divergence between 2 and 6 for an axial distance of at least the initial diameter thereof; one or more inlets disposed tangentially to said chamber at the narrower end of said diverging part for admitting said gas stream with a gyratory motion and thereby separating said gas into a hot fraction which moves as a peripheral stream through said diverging part and a cold fraction forming a core within said peripheral stream; and means for discharging said gas from the chamber.

2. A vortex apparatus according to claim 1 wherein said angle of divergence is between 3 and 4.

3. A vortex tube according to claim l wherein the said diverging part has a length of at least four times the initial diameter thereof.

4. In combination with the vortex apparatus according to claim l, means for circulating a cooling medium adjacently to said diverging part of the chamber in heat exchange relation to said peripheral stream vfor cooling said stream.

5. A vortex tube apparatus according to claim 1 which includes an end wall to said chamber situated near to said inlet and on the side thereof opposite the direction of divergence, and said gas discharge means includes individual outlets for said hot and cold fractions, the former being displaced from the inlet in the direction of divergence by a distance of at least the initial diameter of the diverging part and the latter being a passageway in said end wall situated substantially at the central axis of the chamber and having a diameter less than that of the chamber.

6. A vortex tube for producing cold gas from an initial gas stream under pressure by the heat-separation effect comprising: a tubular body having an interior surface of revolution about an axis; one or more inlets disposed tangentially to said surface and dividing the tube into hot and cold ends situated, respectively, on opposite sides of the inlet, the interior surface of said hot end being empty and divergent in a direction away from the inlet with an angle of divergence between 2 and 6 for a distance of at least the initial diameter thereof; means providing at said cold end a discharge orifice for cold gas; and means for circulating a cooling fluid in heat exchange relation to the outside of the said hot end of the tube.

7. A vortex tube according to claim 6 having a separate outlet for hot gas at the end of the hot end of the tube which is remote from the inlet.

References Cited in the file of this patent UNITED STATES PATENTS 1,952,281 Ranque Mar. 27, 1934 2,519,028 Dodge Aug. 15, 1950 FOREIGN PATENTS 858,260 Germany Dec. 4, 1952 1,090,306 France Mar. 29, 1955 

